While our knowledge on early liver development increased considerably in the past decade, much remains to be learned on how the distinct regulators of liver organogenesis operate at the molecular level. This information is necessary not only to uncover how the function of a particular developmental regulator is required for certain developmental processes, but also to understand how its derailed activity could predispose to liver disease. Our initial studies of the mouse homeobox-gene Prox1 showed that its function is a crucial requirement for early liver morphogenesis and for hepatoblast proliferation (Sosa-Pineda et al., 2000). More recently, we demonstrated that Prox1 function is also necessary for late aspects of liver organogenesis and to maintain the integrity of this organ of adults. In this application, I put forth several molecular and genetic approaches to further our previous studies on the functional role of Prox1 in the developing mouse liver and to uncover how Prox1-expressing endoderm precursors contribute to liver organogenesis. In addition, by taking advantage of two novel mouse models generated in my laboratory I propose to uncover how the Prox1 function is required at postnatal stages for liver homeostasis. After completing the experimental approaches proposed in this application I expect to have gained valuable information on mammalian liver organogenesis and to uncover how certain molecular mechanisms contribute to preserve the integrity of this organ of adults. This information should be relevant to understand the molecular origin of some liver diseases and for studies aimed at differentiating stem into functional liver cells that could be used for therapeutic purposes. RELEVANCE: The basic structure of the liver starts to be established during embryogenesis and is completed at postnatal stages. Uncovering the molecular and cellular mechanisms that govern mammalian liver development should be highly significant to disclose the origin of human congenital liver malformations. Our previous studies of the mouse homeobox-containing Prox1 gene showed that its function is a crucial requirement for early liver morphogenesis. In this application, I put forth several molecular and genetic approaches to further our previous studies on the functional role of Prox1 in the developing mouse liver, to disclose how Prox1 function is required for postnatal liver homeostasis, and to uncover how Prox1-expressing endoderm precursors contribute to liver organogenesis. My long-term expectation is that the information obtained from these studies will be relevant to understand the molecular origin of certain liver diseases in humans.